Different operations are performed during the drilling and completion of a subterranean well, and also during the production of fluids from subterranean formations via the completed well. For example, different downhole operations are typically performed at some depth within the well, but are controlled at the surface.
A perforating process is one type of downhole operation that is used to perforate a well casing. A conventional perforating process is performed by placing a perforating tool (i.e., perforating gun) in a well casing, along a section of the casing proximate to a geological formation of interest. The perforating tool carries shaped charges that are detonated using a signal transmitted from the surface to the charges. Detonation of the charges creates openings in the casing and concrete around the casing, which are then used to establish fluid communication between the geological formation, and the inside diameter of the casing.
Another example of a downhole operation is the setting of packers within the well casing to isolate a particular section of the well or a particular geological formation. In this case, a packer can be placed within the well casing at a desired depth, and then set by a setting tool actuated from the surface. Other exemplary downhole operations include the placement of logging tools at a particular geological formation or depth within the well casing, and the placement of bridge plugs, casing patches, tubulars, and associated tools in the well casing.
One critical aspect of any downhole operation involves ascertaining the depth in the well where the operation is to be performed. The depth is typically ascertained using well logs. A conventional well log includes continuous readings from a logging instrument, and an axis which represents the well depths at which the readings were obtained. The instrument readings measure rock characteristics such as natural gamma ray radiation, electrical resistivity, density and acoustic properties. Using these rock characteristics geological formations of interest within the well, such as oil and gas bearing formations, can be identified. The well is initially logged “open hole” which becomes the bench mark for all future logs. After the well is cased, a cased hole log is then prepared and correlated, or “tied in”, to the open hole log.
Using the logs and a positioning mechanism, such as a wire line or coiled tubing, coupled to an odometer, a tool can be placed at a desired depth within the well, and then actuated as required to perform the downhole operation. One problem with conventional logging and positioning techniques is that it is difficult to accurately identify the depth of the tool, and to correlate the depth to the open hole logs.
FIG. 1 illustrates a prior art perforating process being performed in an oil and gas well 10. The well 10 includes a well bore 12, and a casing 14 within the well bore 12 surrounded by concrete 16. The well 10 extends from an earthen surface 18 through geological formations within the earth, which are represented as Zones A, B and C. The casing 14 is formed by tubular elements, such as pipe or tubing sections, connected to one another by collars 20. In this example the tubular elements that form the casing 14 are about 40 feet long so that the casing collars 20 are forty feet apart. However, tubular elements with shorter lengths (e.g., twenty feet) can be interspersed with the forty feet lengths to aid in depth determinations. Thus in FIG. 1 two of the casing collars 20 are only twenty feet apart.
For performing the perforating operation a perforating tool 22 has been lowered into the casing 14 on a wire line 24. A mast 26 and pulleys 28 support the wire line 24, and a wire line unit 30 controls the wire line 24. The wire line unit 30 includes a drive mechanism 32 that lowers the wire line 24 and the tool 22 into the well 10, and raises the wire line 24 and the tool 22 out of the well 10 at the completion of the process. The wire line unit 30 also includes an odometer 34 that measures the unwound length of the wire line 24 as it is lowered into the well 10, and equates this measurement to the depth of the tool 22 within the well.
During formation of the well 10 an open hole log 36 was prepared. The open hole log 36 includes various instrument readings, such as gamma ray readings 38 and spontaneous potential (SP) readings 40 which are plotted as a function of depth in feet. For simplicity only a portion of the open hole log 36, from about 7000 feet to about 7220 feet, is illustrated. However, in actual practice the entire well 10 from the surface 18 to the bottom of the well 10 may be logged. The open hole log 36 permits skilled artisans to ascertain the oil and gas containing formations within the well 10 and the most productive intervals of those formations. For example, based on the gamma ray readings 38 and the SP readings 40 it is determined that Zone A may contain oil and gas reserves. It is thus desired to perforate the casing 14 along a section thereof proximate to Zone A.
In addition to the open hole log 36, following casing of the well 10, cased hole gamma ray readings 44 are made, and a casing collar log 42 can be prepared. The casing collar log 42 is also referred to as a PDC log (perforating depth control log). The casing collar log 42 can be used to identify the section of the casing 14 proximate to Zone A where the perforations are to be made.
Using techniques and equipment that are known in the art, the casing collar log 42 can be accurately correlated, or “tied in”, to the open hole log 36. However, using conventional positioning mechanisms, such as the wire line unit 30, it may be difficult to accurately place the perforating tool 22 at the required depth within the well. For example, factors such as stretching, elongation from thermal effects, sinusoidal and helical buckling, and deformation of the wire line 24 can affect the odometer readings, and the accuracy of the odometer readings relative to the open hole odometer readings.
Thus, as shown in FIG. 1, the odometer readings which indicate the depth of the perforating tool 22, may not equate to the actual depths, as reflected in the open hole log 36 and the casing collar log 42. In this example, the odometer readings differ from the depths identified in the open hole log 36 and the casing collar log 42 by about 40 feet. With this situation, when the perforating tool 22 is fired, the section of casing 20 proximate to Zone A may be only partially perforated, or not perforated at all.
Because of these tool positioning inaccuracies, various correlative joint logging and wire logging techniques have been developed in the art. For example, one prior art technique uses electronic joint sensors, and electrically conductive wire line, to determine joint-to-joint lengths, and to correlate the odometer readings of the wire line to the casing collar log. Although these correlative joint logging and wire line logging techniques are accurate, they are expensive and time consuming. In particular, additional crews and surface equipment are required, and additional wire line footage charges are incurred.
In addition to tool positioning inaccuracies, computational errors also introduce inaccuracies in depth computations. For example, a tool operator can make computational errors by thinking one number (e.g., 7100), while the true number may be different (e.g., 7010). Also, the tool operator may position the tool by compensating a desired amount in the uphole direction, when in reality the downhole direction should have been used. These computational errors are compounded by fatigue, the weather, and communication problems at the well site.
It would be desirable to obtain accurate depth readings for downhole tools without the necessity for complicated and expensive correlative joint logging and wire logging techniques. In addition, it would be desirable to control down hole operations and processes without having to rely on inaccurate depth readings contaminated by computational errors. The present invention is directed to an improved method and system for performing operations and processes in wells, in which the depths of down hole tools are accurately ascertained and used to control the operations and processes.
Another limitation of conventional downhole operations that are dependent on depth measurements, is that downhole tools must first be positioned in the well, and then actuated from the surface. This requires additional time and effort from well crews. In addition, surface actuation introduces additional equipment and variables to the operations. It would be advantageous to be able to control downhole operations without the requirement of surface actuation of the downhole tools. With the present invention actuation of downhole tools can be performed in the well at the required depth.